Experimental set-up (continued)
Distributed Raman amplification is used to compensate the loss of the four SLA/IDF spans. Semiconductor lasers provide three depolarized Raman pump waves (1440, 1455 and 1487 nm) co-propagating with the signal to give a forward gain of about 7.5 dB and two counter-propagating Raman pump waves (1427 and 1455 nm) provided by fiber lasers give about 18.5 dB of backward gain. The counter-propagating pump waves are coupled to the transmission fibers by circulators to eliminate possible reflections. A fixed gain flattening filter after each SLA/IDF span reduces the peak-to-peak gain ripple per span to less than 1 dB. The net gain of a SLA/IDF span is 3.5 dB to accommodate the losses in the couplers/circulators for the Raman pumps and the gain flattening filters. Figure 2b shows the evolution of the channel power in a SLA/IDF span. The path average power is 5.8 dB higher than without the Raman amplification. Distributed Raman amplification is also used to compensate the loss of the 28 km IDF.
In addition to the transmission fiber, the loop contains a dynamic gain flattening filter and two EDFAs to compensate for the loss in the loop switch and the dynamic gain flattening filter. The WDM signal at the output of the loop switch is post dispersion compensated and then demultiplexed in an AWG so that the selected channel can be sent to the receiver for BER measurement.
A 43 Gbit/s ETDM receiver was used in the experiment. It consists of a photo diode and a 43 Gbit/s clock and data recovery that extracts the 42.657 Gbit/s electrical data signal. This signal is demultiplexed into the four 10.664 Gbit/s tributaries which are subsequently FEC decoded to get the 9.953 Gbit/s PRBS. The measured BER is the average of the four tributaries.
Fig. 2a. The first 1000 km of the dispersion map (the dots indicate start/end points of SLA/IDF spans).
Fig. 2b. The evolution of the channel power throughout a single SLA-IDF-SLA span
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